1. Field of the Invention
The present invention relates to an immersing apparatus for immersing a porous body in liquids in a plurality of stages, which is used, for example, in the production of a glass via a porous body. Also, the present invention relates to a method for producing a glass with the use of the above immersing apparatus according to the sol-gel process.
2. Discussion of Related Art
In the art, various immersing-related methods are generally known, which include one in which pores of a porous glass are stuffed with a metal salt component by immersing the porous glass in a solution (U.S. Pat. Nos. 3,938,974 and 4,302,231), and another in which a porous gel produced according to the sol-gel process and having pores in which a metal salt solution is enclosed is immersed in a solvent in which the metal salt is sparingly soluble to thereby crystallize the metal salt into microcrystals in the pores of the porous gel (J. Non-Crystalline Solids 100 (1988) 506-510). The known methods further include one comprising preparing a silica sol containing at least one metal component, gelling the sol, immersing the wet gel in a solution capable of dissolving or dispersing the above metal component, such as acids, to thereby leach out the metal component capable of providing a refractive index distribution so that the metal component is provided with a concentration distribution, and immersing the gel in a silicon alkoxide solution, followed by drying and sintering (Japanese Unexamined Patent Application Publication 95124/1988).
The above J. Non-Crystalline Solids 100 (1988) 506-510 further describes a method comprising immersing a gel in a solvent having a low surface tension prior to drying to thereby prevent cracking at the time of drying.
As described above, various effects are exhibited by sequentially immersing a porous glass or a porous gel in a plurality of solutions so as to change a solvent system within the pores of the porous glass or porous gel.
Conventionally, the transfer of the immersed porous body from a given solution to a subsequent solution has generally been conducted by picking up the porous body from the given solution with the use of a tool such as a basket or a pair of tweezers and immersing the picked porous body in the subsequent solution placed in a separate vessel.
Moreover, Japanese Unexamined Patent Application Publication 226832/1987 discloses a method comprising leaching a porous body 4 with the use of a vessel 3 provided with an inlet port 1 at a lower part of the vessel and a discharge port 2 at an upper part of the vessel as shown in FIG. 7.
However, the above prior art has the following drawbacks with respect to the handling of the porous body (such as porous glass and gel produced by the sol-gel process) when the immersing of the porous body is conducted in multiple stages.
First, when a strong force is applied to the porous body being transferred, the porous body is likely to suffer from partial collapse or flaw because it is generally brittle. Consequently, the greatest possible care should be taken at the time of the transfer of the porous body. Conventional tools are, for example, tweezers, tongs like the same and baskets. Even if the greatest possible care is taken with the use of such conventional tools, the accident of porous body breakage cannot be avoided although the frequency of porous body breakage accidents may be reduced.
Secondly, when the porous body to be treated in multiple stages is picked out of the solution for transferring the same into the subsequent solution, the solvent of the solution, e.g., alcohol, filling the pores of the porous body having been picked up in the air is rapidly evaporated, so that the porous body suffers from a stress due to the surface tension of the solution within the pores, thereby being likely to be cracked. A measure comprising providing an atmosphere saturated with the solvent over the vessel accommodating the immersing solution would be considered for pre venting the above rapid evaporation of the solvent. However, when the immersing is conducted in multiple stages, the above atmosphere control is very inconvenient and costly.
When a solvent having a low surface tension, such as acetone, methanol and ether, is used as the solvent incorporated in the gel, drying gradually proceeds so that cracking is less likely to occur, as long as the drying speed is suitable. However, this solvent has a low boiling point and thus a high volatility, so that it has a drawback that rapid drying of only the surface of the gel is advanced to thereby cause the gel to suffer from cracking due to a stress applied thereby.
Therefore, the immersing treatment in multiple stages has inevitably been accompanied by the problems including the damaging of the porous body during the holding and transfer thereof and the complexity of the atmosphere control for preventing the cracking due to the solvent evaporation during the transfer of the porous body.
The porous body includes porous glasses produced by the phase separation--heat treatment and the acid leaching treatment, and porous gels produced by the sol-gel process. In particular, the porous gels produced by the sol-gel process have the drawback that the gel skeleton is so weak that cracking is likely to occur.
On the other hand, the method comprising changing the solvent introduced through an inlet port 1 to thereby continuously change the solvent mixing ratio, without taking out the porous body from the solution, by the use of an apparatus (see FIG. 7) as disclosed in, for example, Japanese Unexamined Patent Application Publication 226832/1987, has drawbacks in that the solvent replacement is inevitably slow, and in that, due to the mixing of two different solutions, a large volume of solvent is required for complete conversion to the subsequent solution to thereby incur a high cost.